The large bug is a Wheel Bug, one of the assassin bugs (family Reduviidae and species Arilus cristatus). If you think in terms of insects beneficial to Man and pests of Man, this bug is definitely beneficial as it preys on many “potential pest” insects. The Spotted Cucumber beetle (Diabrotica undecimpunctata) it is stalking is often considered a pest depending upon its numbers and what it is munching on (particularly if its larvae are feeding on corn roots). The beetle appears to be put on guard when the assassin bug’s foot slips jarring the flower. The fastest route away is to run of the plant rather than take time to take flight.
Notice that with the long head and flexible neck, the Wheel Bug has a lot of freedom to wield its beak weapon.
The small bug is an ambush bug which hides in wait of passing prey and uses its raptorial forelegs to catch prey often much larger than it is. The Wheel bug apparently found this one unappealing and let it go after a close examination (do you call this a close encounter of the 3rd kind?). It belongs to the same family as assassin bugs and may be species Phymata pennsylvanica but I am basing that simply on images I found on the Web, I do not have a useful guide to the Heteroptra.
There were a VERY few Goldenrod weeds blooming this late in the season and these were about 2 feet tall rather than the earlier 5 foot ones that bloomed. This probably left the bug very few choices of where to look for a meal. Several of these blooms had a Spotted Cucumber beetle on them and I first saw this Wheel Bug with one of these beetles impaled on its beak.
I was surprised that the assassin bug got its antenna so close to the beetle. It did appear bug’s antenna touching the beetle triggered the beetle’s flight response.
The above video was image stabilized using the warp stabilizer in After Effects. The same frames are displayed below to see the impact of having the breeze move the insects around.